A microscope or the like using a charged particle beam scans a sample with the charged particle beam which the sample is irradiated in two dimensions of the horizontal direction and the vertical direction and detects a secondary signal generated from an irradiation region. The microscope amplifies and integrates the detected signal by an electric circuit and correlates the detected signal with scanning coordinates of the charged particle beam, thereby generating a two-dimensional image.
Here, regarding a device that performs image formation, an image forming method for improving a signal-to-noise ratio (S/N ratio) by integrating a plurality of pieces of two-dimensional image data is known. In PTL 1, PTL 2, PTL 3, PTL 4, and PTL 5, in order to suppress the influence of noise as described above, a technique in which the same imaging region is scanned a plurality of times and adds and averages signals obtained by the scanning is described. By performing adding and averaging, it becomes possible to suppress irregularly occurring noise to some extent.
In PTL 1, a method of controlling a gain of a multiplier for integration computation as an input pixel luminance value largely fluctuates due to the influence of noise is described. In PTL 2, a method in which a plurality of frame memories necessary for frame integration are mounted, a frame image before two frames is also set as a target of integration computation, and a multiplication rate thereof is switched is described. In PTL 3, a frame integration method in which an averaging arithmetic expression of frame integration computation multiplied by an exponent and divided by the exponent is used as an arithmetic expression is described. In PTL 4, a method of appropriately adjusting signal intensity of an image to be integrated is described. In PTL 5, a method of detecting positional deviation and variably setting a frame integration number with respect to the degree of positional deviation is described.